The present invention relates to a liquid crystal display device, more particularly to a technology for improving a display quality of an active matrix display device. The technology employed in this invention is preferred in applying especially to the active matrix display device in which signal wires and common electrode of a display panel are formed in different layers from each other and whose display part is manufactured through divisional light-exposing.
In a liquid crystal display device, images are displayed by applying an electric field to a liquid crystal material. As a method of applying the electric field, there is a static driving method by which a constant voltage signal is constantly applied to each electrode of a display panel. However, this method requires an enormous number of signal wires if a display is large in size. As a result, multiplex driving methods in which the signal voltage is applied by time-sharing are employed in this case. Among the multiplex driving methods, an active matrix method provides a high display quality since an electric charge given to the electrode can be retained until next frame.
Concerning the direction of the electric field to be applied to a liquid crystal material, the method is classified into two modes, i.e., one mode for applying the electric field perpendicular to glass substrates sandwiching the liquid crystal material, and the other mode for applying the electric field in parallel to the substrates (In-plane Switching which is often abbreviated as xe2x80x9cIPSxe2x80x9d). The In-Plane Switching mode is appropriate for use in a large scale monitoring since it can realize a wide field of view in terms of angle.
FIG. 5 shows an electrode structure concerning the pixel of a liquid crystal display device to which the In-Plane Switching mode is applied for driving, e.g., refer to the disclosure of Japanese Patent Kokoku Publication JP-B-63-21907/1988. This reference discloses a liquid crystal display device having a display panel equipped with a pair of substrates. One of the substrates has display electrodes (pixel electrodes) and reference electrodes (common electrodes) thereon, both electrode being formed as comb-shaped electrodes intermeshing each other. The liquid crystal display device is driven by applying an electric field having a component in parallel to the substrate surface of the panel.
Now, the structure of a conventional liquid crystal display device will be explained. FIG. 6 is a plan view showing the whole structure of a liquid crystal display panel 501. Referring to FIG. 6, display parts 504 are connected to leading wires 503. The leading wires 503 are connected to connection terminals 502, respectively. Divisional positions illustrated in FIG. 6 by vertical and horizontal broken lines correspond ideally to the dividing positions between the display parts produced through divided exposure of light within an entire plane.
Namely, the display parts of a liquid crystal display device employing the IPS mode for driving the liquid crystal are prepared by patterning through the divisional light-exposing. The divisional light-exposing in this manner can take the following advantages:
1. Photomasks are of a low price; and
2. A display panel of a large size can be produced. Because the light-exposing area of one shot is limited
FIG. 5 is an enlarged fragmentary plan view showing the vicinity of the divisional positions illustrated in FIG. 6. In a constitutional example illustrated in FIG. 5, the divisional position is at the center of a signal wire 111 or of a scanning wire 101. Setting the divisional line on the center of a signal wire 111 or of a scanning wire 101 in this manner is mainly due to the following advantages:
1. good symmetry;
2. easy design of patterns; etc.
A pixel for display includes scanning wire 101, signal wire 111 and common electrode wire 102, which are connected to an outside driving circuit. The display pixel further includes a switching element of a TFT (Thin Film Transistor) 131 and comb-shaped pixel electrode 112.
FIG. 7 is a cross sectional view taken along line a-axe2x80x2 of FIG. 5. Referring to FIG. 7, the common electrode wires 102 are formed on a glass substrate 113 of the TFT side, and the pixel electrodes 112 as well as the signal wires 111 are formed thereon through an intermediary of an interlaminar insulating film 105. The pixel electrodes 112 and the common electrode wires 102 are arranged alternatingly in parallel. These electrodes are covered with a protective insulating film 106, on which an orientation film 107 for orientating liquid crystal 301 is coated. Then, the top: of the orientation film 107 is treated by rubbing to complete a substrate 114 of the TFT side.
On a glass substrate 203 of a color filter (abbreviated as xe2x80x9cCFxe2x80x9d) side, a black matrix 201 and color layer 202 for color display are formed in this order. Further, on the color layer 202, leveling film 207 for leveling the top of the substrate 203, and orientation film 207 for orientating the liquid crystal 301 are provided in this order. The top of the orientation film 207 is then treated by rubbing in a direction reversed of the rubbing direction of the orientation film 107 of the TFT side.
Thus, a substrate 208 of the color filter side is completed.
Then, the liquid crystal 301 and spacer 302 (e.g., spherical particles) are encapsulated in between both of the substrates 114 and 208. The gap therebetween is determined by a particulate diameter of the spacer 302.
Finally, a polarizing plate 110 of the TFT side is adhered to the surface of the TFT side glass substrate 113 on which no electrode pattern is formed; and a polarizing plate 205 of the CF side., to the surface of the glass substrate 203 on which no pattern is formed. In this process, the polarizing plate 110 is arranged to make a light-transmitting direction (axis) therethrough perpendicular to the rubbing direction of the orientation film 107. The CF side polarizing plate 205 is arranged to make a light-transmitting direction therethrough perpendicular to that of the TFT side polarizing plate 110. A liquid crystal display panel is completed through the above steps.
In the course of forming a pattern of a layered form on the glass substrate 113 of the TFT side, light is exposed area by area to all of the spots (ideally) divided by the divisional positions shown in FIG. 5. Hereinafter, a layer in which common electrode wires 102 and scanning wires 101 are to be formed or formed may be referred as xe2x80x9cG layerxe2x80x9d; a layer in which signal wires 111 and pixel electrodes 112 are to be formed or formed, to xe2x80x9cD layerxe2x80x9d.
Function of the conventional liquid crystal display will be explained as follows.
Referring to FIG. 6, signal voltages applied to connection terminals 502 are input correspondingly through leading wires 503 into scanning wires 101, signal wires 111 and common electrode wires 102, as illustrated in FIG. 5.
When a signal of xe2x80x9cON voltagexe2x80x9d is input through a scanning wire 101, electric charge flows from a signal wire 111 into a pixel electrode 112 through a TFT 131
FIG. 8 shows a time chart of electric potentials applied to the scanning wire 101, signal wire 111 or common electrode wire 102, respectively.
When a potential difference is produced between scanning wire 101, common electrode wire 102 and pixel electrode 112, a lateral,electric field is applied to the liquid crystal layer in parallel to the substrates corresponding to the potential difference. As a result, liquid crystal molecules are turned to be parallel to the substrates. Then, light transmittance is changed correspondingly in the area between the neighboring parallel extending wires, e.g., between the common electrode wire 102 and the pixel electrode 112.
FIG. 9 shows the qualitative relation of potential difference and light transmittance in between common electrode wire and pixel electrode. According to this relation, light transmittance can be appropriately controlled for driving a liquid crystal display device.
A typical conventional method relating to the pole inversion of an electric field applied to the signal wire includes the following two modes.
One is so-called xe2x80x9cgate line inversion drivingxe2x80x9d mode schematically shown in FIG. 10. This mode is for driving a liquid crystal display panel so that lateral one line has always the same polarity. Polarity of the same line is switched at every frame (i.e., even frame after odd frame).
The other is so-called xe2x80x9cdot inversion drivingxe2x80x9d mode schematically shown in FIG. 11 for driving the display panel so that polarity is changed alternately in a checked pattern. Further, the polarity is switched at every frame.
The method of the pole inversion further includes drain inversion driving mode, frame inversion driving mode and the like. The former is for inverting the polarity of the signal wire controlled to have the same polarity line by line at every frame. The latter is for inverting the polarity for every frame having the same polarity over the entire display face.
Among those methods, the dot inversion driving mode is the most advantageous in display quality, since it causes the least flicker and crosstalk in a displayed image. These defects are little worth consideration as compared with those caused by other modes.
However, there are problems encountered in the course of the investigations toward the present invention. Namely, the following image defects will be observed in the liquid crystal display device employing the IPS mode, in case where there is deviation in the precision of the light exposing position, i.e., the edge of exposed light spot is deviated from a divisional position upon forming a wiring pattern through the aforementioned divisional light-exposing:
1. emphasized (sharp) separation line corresponding to the divisional position
2. vertical faded (faint) stripes
3. crosstalk
In a case of the light-exposure deviation (i.e., where an image (solid image of half tone) is intended to be displayed on the screen of a display panel obtained through the step of the divisional light-exposing accompanied with the above displacement from the divisional line), for example, as shown in FIG. 12, image defects such as fluctuation of brightness, crosstalk and the like will appear.
The causes of the above defects will be discussed below taking especially the case of the dot inversion driving mode as an example.
In forming a wiring pattern through the divisional light-exposing, precision capable of coinciding the contour (edge) of the divisionally exposed light spot with the divisional position is in the order of about 0.5 xcexcm. Accordingly, the deviation (displacement) of the light spot contour from the divisional position may be produced in the same extent. As a result, a pattern of the divisionally light-exposed spot may be shifted to right and left or up and down. This shift of the pattern causes a change in the electric characteristics of a display panel to bring about a problem in the optical properties change.
Let""s consider the case of displacing the light spot contour from the divisional position as exemplary illustrated in FIG. 14(a) to shift two of the adjacent G layers on the right and left to each other to the direction that they come near to each other with the divisional position being at the center. In this case, parasitic capacities Ca1, Ca2, Cb1, Cb2, Cc1 and Cc2 between signal wires 111 and their neighboring common electrode wires 102 on both sides thereof are increased or decreased as follows as compared with those in the case of no displacement occurring on the divisionally exposed light spot. The capacities Ca1, Cb1, Cb2 and Cc2 increase, while capacities Ca2 and Cc1 decrease. Consequently, Ca1 +Ca2≈Cc1+Cc2 less than  less than Cb1+Cb2.
Accordingly, the parasitic capacity connected to the signal wire 111 disposed at the divisional line becomes distinctly large as compared with others.
When signals are input into a display panel instructing the display panel to display a solid image by the dot inversion driving mode, electric potentials of two adjacent signal wires would have the same amplitude but opposite polarities to each other. In the ideal case where no displacement of the light exposure appears, the electric potential of the common electrode wires adjacent to each of the signal wires will not be affected by the change in the electric potential of the signal wires because of compensation by+andxe2x88x92. In this contrast, when the displacement of the light exposure occurs as shown in FIG. 12, two of the parasitic capacities in each of right and left elements divided at the center divisional position become asymmetric which are connected to both sides of the signal wire of the divisional position. As a result, the electric potential of the common electrode wires 102 will be affected by the change in the electric potentials of the signal wires.
FIG. 16 shows the relation of the electric potentials of signal wires, common electrode wire and scanning wire in case where the potential of the common electrode wire is affected by the potentials of the neighboring signal wires. The abscissa represents xe2x80x9ctimexe2x80x9d; the ordinate, xe2x80x9cvoltagexe2x80x9d. In case of applying the dot inversion driving mode, the polarity is different at every neighboring element. Namely, when an element has positive (or negative) polarity, the element adjacent thereto has negative (or positive) polarity. As can be seen from FIG. 16, the actually applied (xe2x80x9cwrittenxe2x80x9d) potential difference between the common electrode and the pixel electrode in each of the elements differs from one to another depending on the polarity and the direction of deviation at the time the signal xe2x80x9cON voltagexe2x80x9d is applied to the signal wire to switch the TFT element xe2x80x9cONxe2x80x9d in the case where the potential of the common electrode wire (shown in FIG. 16 by a broken line) is affected (deviated) by the potentials of the signal wires.
The code xe2x80x9cVrightxe2x80x9d or xe2x80x9cVleftxe2x80x9d in FIG. 16 represents the maximum potential difference between a pixel electrode and a signal wire disposed in the right or left element, respectively. The common electrode wire is not independent (separated) for every element so that the potential of the common electrode wire located apart from the divisional position is also affected by the potential of the signal wire to some extent, correspondingly.
As explained in the above, the difference (fluctuation) in brightness on the screen of the display panel is caused depending on the applied polarity (+orxe2x88x92).
This phenomenon is schematically illustrated in FIG. 17. The abscissa represents the xe2x80x9cdisplayed positionxe2x80x9d the ordinate, the xe2x80x9cbrightnessxe2x80x9d. The difference in the brightness becomes maximum at the divisional position where the deviation effect of the potential of the common electrode wire is at the maximum. This difference decreases depending on the time constant of the common electrode wire in proportion to the distance of the displayed position from the divisional position.
This is the reason why the emphasized (sharp) line as a divisional line is observed at the divisional position and the vertical faded (faint) stripes are observed all over the screen of the display panel as schematically illustrated in FIG. 13(a).
In this addition, lateral crosstalk is observed when an image of white or black window as schematically illustrated in FIG. 13(b) is displayed on the screen of the display panel. This is caused by the difference in the extent of deviations of the common signal wires, i.e., between ones generated by the signal wires which are used for display the white or black window and the others which are not used.
On the other hand, when the adjacent right and left G layers shifted in the direction that they come apart from each other as illustrated in FIG. 14(b) with the divisional position being on the center, a relation of Ca1+Ca2≈Cc1+Cc2 greater than  greater than Cb1+Cb2 is established to cause the same phenomenon as explained in the above.
When the light exposure is deviated (displaced) from the divisional position so as to shift two of the adjacent right and left D layers to each other toward the center as illustrated in FIG. 15(a) with the divisional position being on the center, a relation of Ca1+Ca2≈Cc1+Cc2 greater than  greater than Cb1+Cb2 will be established.
When the light exposure is displaced from the divisional position to shift two of the adjacent right and left D layers in the direction that they come apart from each other as illustrated in FIG. 15(b) with the divisional position being on the center, a relation of Ca1+Ca2≈Cc1+Cc2 less than  less than Cb1+Cb2 will be established to cause the aforementioned phenomenon.
When the displacements as illustrated in FIGS. 14(a) and 15(b) or 14(b) and 15(a) occurred simultaneously, the display quality (image quality) deteriorate more distinctly.
All of these faults essentially result from the above displacement which causes the difference in the parasitic capacity between the signal wire 111 and the common electrode wire 102 in the vicinity of and relative to the divisional position. The deterioration of the image quality increases as the number of the divisions increases.
The above phenomena are observed to any extent, large or small, also in the case where other modes than the dot inversion driving mode are applied.
Accordingly, the present invention has been made in consideration of the above problems. It is an object to provide an active matrix liquid crystal display device whose display portion is manufactured by patterning through the divisional light exposure and which is improved in display quality by reducing emphasized separation lines corresponding to the divisional positions, vertical faint stripes and lateral crosstalk resulting from the deviation of the light exposure.
According to one aspect of the present invention, there is provided an active matrix liquid crystal display device comprising signal wires formed in one layer and common electrodes formed in another layer on a display face by patterning through divisional light-exposing. The liquid crystal display device of the present invention is characterized by compensating capacity patterns each provided on the signal wire and/or the common electrode so that a parasitic capacity at the divisional part does not differ from that disposed on other positions remote from the divisional part, said parasitic capacity being generated between a signal wire and a common electrode disposed closest thereto.
According to a second aspect of the present invention, there is provided an active matrix liquid crystal display device, in which a liquid crystal is driven by an electric field component parallel to a substrate surface of a display panel, and which has one layer comprising scanning wires and common electrodes as well as another layer comprising signal wires and pixel electrodes on a substrate of the display panel, both the layers being patterned through divisional light-exposing. The device is characterized by compensating capacity patterns provided on the signal wire and/or the common electrode so as to make parasitic capacity at the divisional part substantially equal to parasitic capacity at positions other than the divisional part in terms of the parasitic capacity between the signal wire and the common electrode wire nearest therefrom.
According to a third aspect, it is preferred that each of the compensating capacity patterns is a branch electrode provided on the signal wire, projected laterally from the longitudinal periphery of the signal wire, extended beyond the common electrode nearest from the signal wire, and then bent to be extended by a predetermined distance parallel to the signal wire.
According to a fourth aspect, it is preferred that each of the compensating capacity patterns is a branch electrode provided on the common electrode, projected therefrom so as to be superimposed with the signal wire when viewed from the top and extended by a predetermined distance along the longitudinal direction of the signal wire.
According to a fifth aspect, there is provided an active matrix display device, in which a liquid crystal is driven by an electric field having a component parallel to a substrate surface of a display panel, on one substrate of which one layer comprising scanning wires and comb-shaped common electrodes as well as another layer comprising signal wires perpendicular to the scanning wires and pixel electrodes, the two layers being patterned through divisional light-exposing, at least the signal wires being periodically on the divisional position. The display device is characterized by the following features:
(a) that branch electrodes, each of which is projected from one or both longitudinal peripheries of the signal wires, extended beyond a comb-shaped portion of the common electrode nearest therefrom, and then bent to extend a portion by a predetermined distance parallel to the signal wire,
(b) that in case where the signal wire on the divisional position is deviated upon the light-exposing, parasitic capacity on the divisional position is made substantially equal to parasitic capacity at positions other than the divisional position by decreasing/increasing parasitic capacity at the divisional position between the bent and extended portion of the branch electrode and the nearest comb-shaped portion of the common electrode according to increasing/decreasing the parasitic capacity at the divisional position between the branch electrode and the comb-shaped portion of the common electrode to make the sum of the parasitic capacities invariable.
According to a sixth aspect of the present invention, there is provided an active matrix display device, in which a liquid crystal is driven by an electric field having a component parallel to a substrate surface of a display panel, and which has one layer comprising scanning wires and comb-shaped common electrodes as well as another layer comprising signal wires perpendicular to the scanning wires and pixel electrodes on one substrate of the display panel, the two layers being patterned through divisional light-exposing by assuming divisional positions at least on said signal wires. The display device is characterized by the following features:
(a) that auxiliary electrode patterns are arranged, each of which is projected with a predetermined width from the common electrode to be superimposed with the signal wire when viewed from the top and extended by a predetermined distance along the longitudinal direction of the signal wire,
(b) that in case where the signal wire on the divisional position is deviated upon the light-exposing, parasitic capacity on the divisional position is made substantially equal to parasitic capacity at positions other than the divisional position by decrease/increase of parasitic capacity at the divisional position between the signal wire and the auxiliary electrode caused by decreasing/increasing the overlapped area of the same signal wire and the same auxiliary electrode according to increasing/decreasing parasitic capacity at the divisional position of between the signal wire and the comb-shaped portion of the common electrode to make sum of the parasitic capacities invariable.
According to a seventh aspect, there is provided an active matrix display device which comprises:
(a) pixels each composed of a pixel electrode, a common electrode wire and an active element; scanning wires and signal wires disposed on a first transparent substrate provided with a liquid crystal orientation film disposed directly or through an intermediary of an insulating layer;
(b) a liquid crystal layer sandwiched between the. first transparent substrate and second transparent electrode disposed opposite each other through an intermediary of the liquid crystal orientation film;
(c) each of the electrodes being adapted to apply an electric field to the liquid crystal layer substantially in parallel to the substrates;
(d) the pixels each being connected to an outside control means for arbitrarily controlling an electric field to be applied depending on patterns to be displayed; and
(e) polarization means for changing a polarized state of an incident light transmitting through one of the substrates,
(f) the display device being manufactured by a process comprising a step of patterning the pixel electrodes, the common electrode wires and the scanning wires through divisional light-exposing,
(g) wherein capacity patterns are compensated to make parasitic capacities between the signal wire and the common electrode wire substantially equal to each other in the whole region of the display face even upon overlapping deviation produced in the light-exposing step between a layer comprising the scanning wires and the common electrode wires and a layer comprising the signal wires and the pixel electrodes.
Preferably, each of the compensating capacity patterns is composed of a branch electrode attached to the signal wire.
Also each of the compensating capacity patterns may be composed of a branch common electrode added to the common electrode wire.
Preferably, the compensating capacity patterns are composed of a branch electrode added to the signal wire and a branch common electrode added to the common electrode wire.